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| United States Patent |
5,087,269
|
|
Cha
, et al.
|
February 11, 1992
|
Inclined fluidized bed system for drying fine coal
Abstract
Coal is processed in an inclined fluidized bed dryer operated in a
plug-flow manner with zonal temperature and composition control, and an
inert fluidizing gas, such as carbon dioxide or combustion gas. Recycled
carbon dioxide, which is used for drying, pyrolysis, quenching, and
cooling, is produced by partial decarboxylation of the coal. The coal is
heated sufficiently to mobilize coal tar by further pyrolysis, which seals
micropores upon quenching. Further cooling with carbon dioxide enhances
stabilization.
| Inventors:
|
Cha; Chang Y. (Golden, CO);
Merriam; Norman W. (Laramie, WY);
Boysen; John E. (Laramie, WY)
|
| Assignee:
|
Western Research Institute (Laramie, WY)
|
| Appl. No.:
|
563226 |
| Filed:
|
August 3, 1990 |
| Current U.S. Class: |
44/626; 34/370; 44/501 |
| Intern'l Class: |
C10L 009/08; F26B 003/08 |
| Field of Search: |
44/501,620,626
34/57 R,57 C,22
|
References Cited
U.S. Patent Documents
| 708604 | Sep., 1902 | Welch | 34/57.
|
| 3755912 | Sep., 1973 | Hamada et al. | 34/57.
|
| 4031354 | Jun., 1977 | D'Souza | 34/57.
|
| 4249909 | Feb., 1981 | Comolli | 44/626.
|
| 4495710 | Jan., 1985 | Ottoson | 44/626.
|
| 4725337 | Feb., 1988 | Greene | 44/626.
|
Primary Examiner: Dees; Carl F.
Attorney, Agent or Firm: Mingle; John O.
Goverment Interests
This invention was made with Government support under DE-AC21-87MC24268
awarded by the Department of Energy. The Government has certain rights in
this invention.
Parent Case Text
This invention represents a continuation-in-part of Ser. No. 07/332,138,
filed Apr. 3, 1989 and now abandoned, entitled Drying Fine Coal in an
Inclined Fluidized Bed, the disclosure of which is herein incorporated by
reference.
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to a process using an inclined fluidized bed
for drying and stabilizing coal fines in an environmentally acceptable and
safe manner to improve heating value and handling characteristics.
2. Background
Coal is dried for a variety of reasons, such as to save on transportation
costs, to increase the heating value, to increase the net dollar value, to
prevent handling problems caused by freezing weather, to improve coal
quality particularly when used for coking, briquetting, and producing
chemicals, to improve operating efficiency and reduce maintenance of
boilers, and to increase coke oven capacity. However, drying of coal
causes increased dust formation as the dry coal is more friable. Further,
reabsorption of moisture must be considered a potential problem.
Dry coal is generally preferred in many coal operations. In World War II
the Germans determined that dry coal improved pyrolysis in Lurgi-Spulgas
ovens, while the French found that the capacity of coking ovens could be
increased by using said coal. Thus, increased tonnages of dry coal were
being sold in the United States up to the 1970's when stringent emission
standards elevated its cost to an uneconomic level.
Another trend in the coal mining industry was its increased mechanization
resulting in an increased percentage of coal fines. Because coal fines
have a greater relative surface area, they are very susceptible to water
absorption. In order to market such fines, drying was necessary.
Difficulties in coal drying abound. Besides the stringent emissions
standards adding an economic burden, numerous explosions and fires have
occurred when low-cost air is employed as the drying medium. Coal dust
fines are more susceptible to dust explosions than are larger particles
(Hertzberg et al., "Domains of Flammability and Thermal Ignitability for
Pulverized Coals and Other Dusts: Particle Size Dependences and
Microscopic Residue Analysis," 18th International Symposium on Combustion
Proceedings, Pittsburgh, Penn, 1982). Often dry coal is treated with heavy
oil before shipping to prevent dust formation and the reabsorption of
moisture.
Many proposed processes for upgrading coal involve fine grinding and
separations in liquids media. The resulting cleaned coal is difficult to
handle using conventional techniques because of fine particles and high
moisture contents. Additional drying is sometimes employed; however,
moisture reabsorption, dust formation with its fire and explosion hazards,
and spontaneous heating often result in unstable products.
Typical processes include that of Greene, U.S. Pat. No. 4,725,337, which
discloses a process for drying and removing impurities from low rank coal
and peat by subjecting the coal to a recycled superheated gaseous medium
to desorb the moisture from the coal and produce superheated gases.
Another is McMahon, U.S. Pat. No. 4,304,571, which discloses a method for
increasing the Btu-value of a solid fuel, for instance, coal, by
subjecting it to hydrothermal treatment in the presence of an added
decarboxylation catalyst, such as soluble salts of vanadium, copper,
nickel or other similar metal. Ruyter et al., U.S. Pat. No. 4,285,140,
uses a process for dewatering and upgrading low rank coal by heating a
pressurized mixture of coal and water at 150.degree.-300.degree. C. After
the water is separated, the coal is further heated to
300.degree.-400.degree. C. under pressure to vaporize additional moisture.
Ottoson, U.S. Pat. No. 4,495,710, discloses a process for the rapid
fluidized bed heating of coal to mobilize tar with subsequent cooling
using a recycle stream. Comolli, U.S. Pat. No. 4,249,909, discloses a hot
gas, fluidized bed wicking up process where coal hydrocarbons prevent
moisture reabsorption.
The general problem of coal drying represents removing three types of
moisture: free, physically bound, and chemically bound. Free moisture is
found in the very large pores and interstitial spaces of coal and maybe
removed by mechanical means as it exhibits the normal vapor pressure
expected of water at that temperature.
Physically bound moisture is more difficult to remove as it is held tightly
in small coal capillaries and pores. Because of this, its vapor pressure
and specific heat are reduced over that expected of free moisture.
Chemically bound moisture is characterized by a bonding between surfaces
and water. Monolayer and multilayer bonding are commonly identified.
Sometimes a fourth type of moisture is identified which comes from the
decomposition of organic compounds. It is really not moisture held in coal
but is produced during coal decomposition.
Coal drying can be characterized by typical drying curves that exhibit
distinct rate regions. Firstly, a transient region occurs as equilibrium
conditions are sought while the material heats. This is followed by a
largely constant rate portion of drying where the material temperature is
relatively constant during the unbound moisture removal, and the drying
rate is generally determined from only the particle size and moisture
content, be it coal or some other material.
The final region is a period of decreasing rate as the material temperature
increases and the physically and chemically bound moisture is removed. For
this drying regime the particle size, temperature, and residence time are
important parameters. Often the drying rate becomes diffusion controlled,
and since diffusivity increases with temperature, higher temperatures are
employed to continue drying the materials. Because coal needs to be
ideally dried to a very low moisture content, appropriate design for
operating in this diffusion controlled region is important.
During the constant rate period, the heat and mass transfer rates are
directly proportional to the driving forces of temperature gradient and
humidity gradient respectively; the appropriate proportionality constants,
however, are usually experimentally determined. Maintaining near maximum
values of said gradients become important when effective drying equipment
is designed.
Adding oil to dry coal is a common method to prevent moisture reabsorption
and autogenous heating. Thus, using 1.5 to 2.0 gallons of No. 6 oil per
ton of coal has been shown to be effective for this purpose (Bauer,
"Thermal Drying of Western Coal--A Review Paper," Western Regional
Conference on Gold, Silver, Uranium, and Coal Proceedings, Rapid City, SD,
September 1980). Processes such as oil addition, however, increase
operating costs.
Willson et al., "Low-Rank Coal Slurries for Gasification," Fuel Processing
Technology, 1987, 15: 157-172, describe a variety of drying techniques to
upgrade low rank coals. Included were hot water and steam drying under
pressure and hot-gas drying using a rotary kiln, Roto-Louvre dryer or a
Perry turbulent entrainment dryer. In this study two bituminous coals,
Illinois No. 6 and Pittsburgh No. 8, and Wyoming subbituminous coal were
employed. When dried directly in hot gases, the dried coal reabsorbs
moisture and returns to nearly the original equilibrium moisture level. In
contrast, both steam and hot-water drying produced dried coal in which
moisture reabsorption was significantly reduced. At these drying
temperatures, 270.degree.-330.degree. C., and under pressure, it was
concluded that residual tar in the dried coal significantly helped in
reducing the moisture reabsorption. However, the high energy requirements
will likely rule out this process for drying ultra-fine, modern-mined
coal.
Ultra-fine coal adds two additional problems to any effective thermal
drying processes--fines carryover and explosions. Since indirect heating
is inefficient as it requires large heat transfer surfaces with a separate
heating medium that escalate capital costs, and leads to high maintenance
requirements and low throughput, an inert atmosphere is needed with a low
gas velocity.
Smith, U.S. Pat. No. 4,170,456, discloses a method for inhibiting the
spontaneous combustion of coal char by treating with carbon dioxide to
deactivate the char surface to oxygen. The temperature ranged used was
10.degree.-149.degree. C. Since coal char and dried coal are similar, this
carbon dioxide treatment would likely reduce the pyrophoric nature of
dried coal.
After World War II fluidized bed dryers were adapted to coal drying;
however, critical control of both coal and gas flow was required in order
to avoid fires and explosions. McNally Flowdryer, Dorr-Oliver Fluo-Solids
Dryer, Link-Belt Fluid Flow Dryer, and Heyl and Patterson fluidized bed
dryers are all well known.
Typically fluidized bed dryers have a coal-fired zone, using stokers or
pulverized coal pneumatically injected, where fluidizing air is heated and
its oxygen content reduced. Another zone acts as the dryer where the
pressure drop across the gas distributor is large relative to the pressure
drop across the bed in order to assure good dryer gas distribution. In
some installations, gas from the coal is recycled to further reduce the
oxygen concentration. Coal distribution is controlled by a feeder-spreader
device, such as a roll feeder, multiple screw feeders, or grate.
These fluidized bed dryers are potentially hazardous when air or mixtures
of air and recycled gas are employed. The oxygen concentration is critical
to avoid explosive conditions, and special safety equipment, such as
sprinkler systems, blowout doors, and automatic fail-safe shutdown
devices, is common. Additionally, the moisture content of the dry coal is
often held to 5-10%, or 0.5-1.0% surface water, to make the drying
operation less hazardous and to avoid excessive formation of dust. After
removal of the surface water, the rising bed temperature becomes the
control parameter to keep it safely below auto-ignition conditions.
Equipment to control particulate emissions from fluidized beds include
combinations of cyclones, electrostatic precipitators, bag filters, and
wet scrubbers. Cyclones are ineffective with particle sizes below five
microns, so their operation is usually restricted to extraction of large
particle dust loading prior to removal of fine dust particles by
subsequent equipment. However, cyclones employed at the gas stream dew
point or with water-spraying, can be nearly as effective as wet scrubbers.
Electrostatic precipitators when successfully used must be kept free of
condensation, and in addition, are subject to malfunctions and frequent
maintenance.
Flash dryers use entrained fluidized beds to dry particles under residence
times of one second or less. This short residence time gives a high
capacity with a low inventory of coal, and makes them less hazardous than
conventional fluidized bed dryers. However, particle fines entrainment due
to the required high gas velocity is a problem, and requires additional
separation equipment.
Conventional dryers, such as Multi-Louvre and Cascade, use many flights and
vibrating shelves to control coal flow in the dryer. With these,
maintenance is a major cost when compared to fluidized bed dryers.
Roto-Louvre is a variation on a rotary drum dryer.
Modern development is exploring a number of technologies to improve coal
drying processes. Hot water dewatering and decarboxylation both employ a
high pressure treating reactor for altering coal micropore structures to
prevent moisture reabsorption, but then additional drying becomes
necessary.
Vapor recompression principles can reduce energy requirements by
compressing water vapor to a higher pressure so that recycle heating can
be employed. In essence much of the heat of vaporization of the water
removed from the coal can be recovered. Pilot plant testing has been
employed but high capital and maintenance costs are a definite drawback.
The multistage fluidized bed process achieves good thermal efficiency by
recompressing water vapor from the first stage and using it to heat and
fluidize the second stage. A portion of the first-stage water vapor is
recycled to fluidize the bed while steam tubes provide heating.
Solar drying processes use a slurry of coal that is pumped to shallow
ponds. The coal then is stockpiled for further air drying. The slurry
requires large amounts of water and ponds require large amounts of land.
The process is effective only in dry climates.
The Fleissner process, developed in 1927, dries coal by heating with high
pressure steam. High steam temperatures change the coal structure and
release water and carbon dioxide leaving a hydrophobic coal remaining for
final drying. However, high steam pressures require elevated capital
costs.
The Koppelman process heats coal some 400.degree. C. above evaporative
drying conditions so that partial pyrolysis occurs releasing oil; this
process requires, however, extensive water cleanup because of the
pyrolysis. The product coal can be almost completely dried, but hot water
is typically used to cool the coal so approximately 5% water is present in
the final product. This process produces enhanced heating value coal, so
potentially longer transportation costs can be economically tolerated.
Unfortunately, extruders are required because of the high pressure and
this is a severe economic disadvantage.
Existing coal dryers can be grouped into three basic types: fluidized bed,
entrained bed or flash, and shallow moving bed. The later can be further
subdivided into Multi-Louvre, vertical tray and Cascade, continuous
carriers, and drum type. McNally Flowdryer, Link-Belt Fluid-Flo dryer,
Heyl, Patterson fluid bed dryer, and Dorr-Oliver Fluo-Solids dryer all use
fluidized beds with hot air or hot gases. Flash dryers, for instance
Combustion Engineering's type, use entrained bed drying with hot gas.
Dryers using a shallow bed are Link-Belt Multi-Louvre, McNally fine coal
Cascade, McNally Vissac, and Link-Belt Roto-Louvre.
SUMMARY OF THE INVENTION
The present invention has several objectives; they include overcoming the
deficiencies of the aforementioned prior art, providing an improved
process for drying coal including coal fines, providing an improved
process for upgrading coal, providing coal which is not subject to
spontaneous combustion, and providing dried coal which does not readily
reabsorb moisture.
Coal is processed in an inclined fluidized bed dryer with staged or zonal
temperature control. The inert fluidizing gas is largely carbon dioxide in
later treatment stages, but may be contain other combustion products is
earlier stages. The carbon dioxide, which is ideally recycled, is produced
by partial decarboxylation of the coal. The coal is heated sufficiently to
mobilize coal tar by pyrolysis, which seals micropores upon quenching with
carbon dioxide to enhance stabilization.
Claims
We claim:
1. A process for the drying and stabilizing of fine coal comprising:
employing a zonal inclined fluidized bed containing coal and using an inert
fluidizing gas;
means for feeding coal;
means for selectively heating said gas;
means for rapidly quenching said fluidized bed; and
means for collecting products.
2. The process according to claim 1 wherein said zonal inclined fluidized
bed further comprises operating with an inclination angle of from zero to
about 15 degrees.
3. The process according to claim 1 wherein employing said zonal inclined
fluidized bed further comprises using multiple one-zone inclined fluidized
beds.
4. The process according to claim 1 wherein said means for feeding coal
further comprises using a zonal inclined fluidized bed.
5. The process according to claim 1 wherein said means for feeding coal
further comprises using coal containing fines.
6. The process according to claim 1 wherein said means for feeding coal
further comprises employing mechanical equipment.
7. The process according to claim 1 wherein said inert fluidizing gas
further comprises substantially carbon dioxide.
8. The process according to claim 1 wherein said inert fluidizing gas
further comprises recycle carbon dioxide from coal pyrolysis.
9. The process according to claim 1 wherein said inert fluidizing gas
further comprises combustion gas.
10. The process according to claim 1 wherein said inert fluidizing gas
further comprises employing near minimum fluidization velocities.
11. The process according to claim 1 wherein said means for selectively
heating said gas further comprises partial pyrolysis of said coal.
12. The process according to claim 11 wherein said partial pyrolysis
further comprises producing substantially carbon dioxide as the gaseous
product.
13. The process according to claim 11 wherein said partial pyrolysis
further comprises producing minute amounts of liquid tars remaining in the
micropores of said coal.
14. The process according to claim 1 wherein said means for selectively
heating said gas further comprises employing a gas plenum providing for
multiple separately heated fluidizing gas inlets.
15. The process according to claim 1 wherein said means for selectively
heating said gas further comprises using multiple internal heaters
selectively positioned within each said fluidized bed zone.
16. The process according to claim 1 wherein said means for selectively
heating said gas further comprises producing near bone-dry product coal.
17. The process according to claim 1 wherein said means for rapidly
quenching said fluidized bed containing coal further comprises employing
cooled inert gas.
18. The process according to claim 15 wherein said cooled inert gas further
comprises employing cooled fluidizing gas.
19. The process according to claim 1 wherein said means for rapidly
quenching said fluidized bed further comprises stabilizing said product
coal against moisture reabsorption.
20. The process according to claim 1 wherein said means for rapidly
quenching said fluidized bed further comprises stabilizing said product
coal against reheating hazards.
21. The process according to claim 1 wherein said means for product
collection further comprises employing a stabilized dried coal transfer
system.
22. The process according to claim 21 wherein said stabilized dried coal
transfer system further comprises employing a fluidized bed.
23. The process according to claim 21 wherein said stabilized dried coal
transfer system further comprises employing a briquetting operation.
24. The process according to claim 21 wherein said stabilized dried coal
transfer system further comprises employing a bagging operation.
25. A process for the drying and stabilizing of fine coal comprising:
employing a zonal inclined fluidized bed using a coal feeder and an inert
fluidizing gas;
means for selectively drying said coal;
means for selectively pyrolyzing said coal;
means for rapidly quenching said coal; and employing a product coal
transfer system.
26. The process according to claim 25 wherein said zonal inclined fluidized
bed further comprises operating with inclination angles of from about 3 to
15 degrees.
27. The process according to claim 25 wherein said zonal inclined fluidized
bed further comprises operating under plug flow conditions.
28. The process according to claim 25 wherein said coal feeder further
comprises using a zonal inclined fluidized bed.
29. The process according to claim 25 wherein said coal feeder further
comprises using mechanical means.
30. The process according to claim 25 wherein said coal feeder further
comprises designing for high moisture coal feed.
31. The process according to claim 25 wherein said inert fluidizing gas
further comprises substantially carbon dioxide.
32. The process according to claim 31 wherein said carbon dioxide further
comprises recycled carbon dioxide from pyrolysis of coal.
33. The process according to claim 25 wherein said inert fluidizing gas
further comprises combustion gas.
34. The process according to claim 25 wherein said zonal inclined fluidized
bed further comprises using a divided inlet gas plenum allowing different
temperature gas streams to fluidize said coal.
35. The process according to claim 34 wherein said different temperature
gas streams further comprises external heating.
36. The process according to claim 34 wherein said different temperature
gas streams further comprises internal heating within said plenum.
37. The process according to claim 34 wherein said different temperature
gas streams further comprises internal heating within said fluidized coal
bed.
38. The process according to claim 25 wherein said means for selectively
drying said coal further comprises reaching a fluidized coal temperature
of about 250.degree. C.
39. The process according to claim 25 wherein said means for selectively
drying said coal further comprises producing product coal dried to below
about three percent moisture content.
40. The process according to claim 25 wherein said means for selectively
pyrolyzing said coal further comprises reaching a fluidized coal
temperature of about between 250.degree. C. and 350.degree. C.
41. The process according to claim 25 wherein said means for selectively
pyrolyzing said coal further comprises producing substantially carbon
dioxide.
42. The process according to claim 25 wherein said means for selectively
pyrolyzing said coal further comprises producing sufficient liquid
pyrolysis tars to approximately obstruct the micropores of said coal.
43. The process according to claim 25 wherein said means for rapidly
quenching said coal further comprises solidifying liquid coal tars within
the micropores of said coal.
44. The process according to claim 25 wherein said means for rapidly
quenching said coal further comprises stabilizing said coal against
spontaneous combustion and moisture reabsorption.
45. The process according to claim 25 wherein said means for rapidly
quenching said coal further comprises cooling with substantially carbon
dioxide below a temperature of 250.degree. C.
46. The process according to claim 45 wherein said carbon dioxide further
comprises filling the micropores of said coal against moisture and oxygen
penetration.
47. The process according to claim 25 wherein said product coal transfer
system further comprises using mechanical bagging.
48. The process according to claim 25 wherein said product coal transfer
system further comprises using briquettes.
49. The process according to claim 25 wherein said product coal transfer
system further comprises employing a zonal inclined fluidized bed.
50. A process for the drying and stabilizing of fine coal comprising:
employing a three zone inclined fluidized coal bed with carbon dioxide as
the fluidizing medium;
using zone one for drying said coal;
using zone two for partial pyrolysis of said coal;
using zone three for rapid quenching of said coal; and
employing a product coal collector.
51. The process according to claim 50 wherein said zonal inclined fluidized
bed further comprises operating at about 5 degrees inclination.
52. The process according to claim 50 wherein said coal bed further
comprises feeding coal with fines.
53. The process according to claim 50 wherein said carbon dioxide further
comprises being recycled from fluidized coal pyrolysis.
54. The process according to claim 50 wherein said zone one further
comprises heating said fluidized coal to about the range 200.degree. to
250.degree. C.
55. The process according to claim 50 wherein said zone two further
comprises heating said fluidized coal to about 350.degree. C.
56. The process according to claim 50 wherein said zone three further
comprises quenching said fluidized coal to about below 200.degree. C.
57. The process according to claim 50 wherein said zone one further
comprises producing coal that is dried to about below one percent moisture
content.
58. The process according to claim 50 wherein said zone two further
comprises producing a gas product of substantially carbon dioxide.
59. The process according to claim 50 wherein said zone two further
comprises producing mobile liquid tars within said coal micropore space.
60. The process according to claim 50 wherein said zone three further
comprises solidifying said tars blocking said coal micropore space to
stabilize the product coal by prohibiting reabsorption of moisture and
oxygen.
61. The process according to claim 50 wherein said zone three further
comprises filling said coal pore space with carbon dioxide to stabilize
the product coal by preventing reheating and allow safe handling.
62. The process according to claim 50 wherein said product coal collector
further comprises bagging.
63. The process according to claim 50 wherein said product coal collector
further comprises briquetting.
64. The product produced by the process of claim 1.
65. The product produced by the process of claim 25.
66. The product produced by the process of claim 50.
Description
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a typical coal drying process employing inclined fluidized
beds.
FIG. 2 shows in two views 2A and 2B a typical inclined fluidized bed bench
scale equipment.
FIG. 3 shows the particle size distribution of tested crushed feed coals.
FIG. 4 shows experimental TGA weight loss curves for heating Usibelli coal.
FIG. 5 shows experimental TGA weight loss curves for heated Eagle Butte
coal.
FIG. 6 shows inclined fluidized bed cold flow experimental results using
Eagle Butte coal.
FIG. 7 shows inclined fluidized bed cold flow experimental results using
Usibelli coal.
FIG. 8 shows moisture and temperature conditions during a typical larger
test run.
DETAILED DESCRIPTION OF INVENTION
The present invention represents a process to thermally dry fine coal to
produce a low-moisture product that is stabilized against moisture
reabsorption, dust formation, and spontaneous combustion. Thus, the
shipping weight is reduced and further surface treatment is unnecessary.
The unique control capabilities of the inclined fluidized bed allow
efficient operation of such process.
According to the preferred embodiment of present invention, recycled carbon
dioxide, produced from partial decarboxylation of coal and representing an
inert gas, dries fine coal to a low moisture content. An inclined
fluidized bed operating at plug flow conditions provides excellent
gas-solid contact while minimizing elutriation from the dryer. The plug
flow nature of the inclined fluidized bed allows drying, tar mobilization,
quenching, and cooling to occur in separate zones by control of the
appropriate reactor temperature profile and solids residence time; thus
producing a zonal inclined fluidized bed. The tar mobilization and
subsequent quenching with carbon dioxide seals off the micropores so that
moisture reabsorption is prevented. The final cooling with carbon dioxide
avoids autogenous heating and leaves the product dried coal in a
stabilized form so that further transfer can be simply done, such as
pressing into briquettes for easy handling and shipping.
FIG. 1 shows a typical block flow sheet for the process showing the
preferred embodiment. The process begins with feed coal, 1, which usually
is predried if the initial moisture content is over 30%. Predrying avoids
mechanically feeding difficulties entering the first inclined fluidized
bed (IFB), 2. This coal passes through the first IFB, 2, and is fluidized
by hot carbon dioxide, 3, entering its bottom plenum, 4. The exit gases,
5, from the first IFB, 2, are treated to remove fines, 6, and then cooled
to remove water, 7, before the gas is compressed by blower action, 8. This
gas stream, 9, now essentially carbon dioxide, is recycled, 10, back to
the plenum of the second IFB, 11. The second IFB, 11, is fed by dried
coal, 12, exiting from the first IFB, 2. As the dried coal exits the
second IFB, 12, as product, it is briquetted, 13, before storage. Part of
the gas stream, 14, exiting the second IFB, 12, flows directly to the
first IFB fluidizing gas plenum, 3. The remaining off-gas from the second
IFB flows through a heat exchanger, 15, in the coal combustor, 16, for
heating before re-entering the inlet plenum stream, 3. The coal combustor
is fed coal fines, 17, that maybe recycled from the fines removal
equipment, 6, and combustion air, 21. The resulting combustor stack gases,
18, and ash, 19, are produced for disposal and in particular this flue gas
is environmentally acceptable as is. Some excess carbon dioxide may be
vented, 20, if leaks in the system do not compensate for the needed carbon
dioxide produced in the first IFB, 2.
In an alternate formulation, the combustor gas, 18, may be employed as part
of the dryer gas, 3, going to the first IFB, 2. Further, it may be used as
the gas for a predryer, if employed.
In a further alternate formulation, the carbon dioxide, 10, needed as the
input fluidizing gas for the second IFB can be obtained from bottled
sources heated to acceptable inlet conditions; thus, recycle is not
employed, and all the gas is vented, 20. In this situation, which is
common for small bench-scale operation, water removal, 7, is not employed
and compression of the gases, 8, is not needed since the bottle gas is at
sufficient pressures to operate the system.
In a further alternate formulation, the product, 13, is not briquetted, but
the dry fine coal is stored for further use, shipped via transportation
equipment, or utilized directly, such as for a coal-fired power plant.
The equipment is standard except for the inclined fluidized beds, 2, and
11. FIG. 2B shows a typical drawing of an inclined fluidized bed scaled to
bench operation. Main characteristics are the lower gas plenum, 25,
although shown using the same inlet gas, 26, can use different gas streams
along the bed length. A further optional feature could be independently
controlled heaters in each inlet gas zone for necessary temperature
control. Similarly, the exit gas stream, 27, is collected into one stream,
but can be kept separate if desired. The design of the exist gas plenum
chamber, 28, FIG. 2B is purposely to keep the pressure drop constant so
that horizontal mixing of the gas fluidizing stream is minimized; thus,
separate exit gas streams of different compositions are possible to
collect. Further this upper plenum area, 28, is by design widened with
multiple exit apertures, 32, to reduce the gas velocity and allow a
disengaging space for larger entrained particles to remain in the bed
region. The inlet coal, 29, enters the bed and moves approximately
horizontal in plug flow as a shallow bed to the discharge position, 30,
efficiently contacting the gas fluidizing stream. The inclination angle of
the bed is measured from the horizontal inlet toward the outlet and is
normally expressed as a positive angle in degrees. The shallow bed height
can be generally controlled by the discharge baffle height, 31. This
shallow bed keeps the concentration of the contacting gas essentially
constant and maximizes the temperature and humidity gradients for
efficient dryer operation. The plug flow prevents undesirable back-mixing.
The velocity of the fluidizing gas is desirably kept at or slightly below
that needed for minimum fluidization to reduce solids entrainment and to
produce the desirable plug flow operation. The residence time of the
material depends upon the slope of the installed inclined fluidized bed,
the feed rate, and the velocity of the fluidizing gas. In the drying of
coal, these appropriate parameters can be experimentally determined such
that the coal product has the desired characteristics. Scaling the size of
inclined fluidized beds is straight-forward because of its simple design.
The two inclined fluidized beds are used for convenience, and the residence
time of the coal for the system is determined by which bed is most
critical. In most designs, the first inclined fluidized bed determines the
system residence time for these beds since its operating parameters are
more critical. It is possible to use only one inclined fluidized bed if
the inlet gas plenum is divided so that cool carbon dioxide can be
employed in the final zone which then serves as cool-down region for the
processed coal. This is referred to as a zonal inclined fluidizing bed.
The inclined fluidized bed serves as a dryer, reactor, and cooler for the
processed coal. The fluidization of the coal particles allows efficient
heat and mass transfer between the solid surface and the bulk gas phase.
The equipment is operated in a plug-flow regime in order to effectively
serve as a dryer. The shallow fluidized bed along with gas cross flow
provides maximum humidity gradient for high mass transfer rates and allows
minimum fluidization gas velocity to reduce carry-over fines to a minimum.
The reactor zone of the inclined fluidized bed performs the decarboxylation
and partial coal pyrolysis reactions where carbon dioxide for recycling is
produced while mobilizing coal tars. The residence time is short along
with a high heating rate to maximize tar production among the many
possible pyrolysis reactions. Next, a rapid cooling of the coal occurs
with exposure to lower temperature carbon dioxide, and serves to quench
the tar in the coal micropores to prevent future moisture reabsorption and
spontaneous combustion.
The inert gas medium during this process is carbon dioxide in order to
prevent explosions of ultra-fine coal and spontaneous combustion of dried
coal. Further, with this final treatment the coal is left with carbon
dioxide in its internal pore space. This helps to prevent moisture from
reentering the pores and to exclude oxygen. Because the moisture
reabsorption is exothermic, any oxygen present tends to enhance the
potential for spontaneous combustion; thus, maintaining a carbon dioxide
internal pore gas requirement prevents the conditions needed for
spontaneous combustion.
Another advantage to this system is that the stabilized dried product coal
is in excellent condition to briquette for easier handling. The operation
for forming briquettes, which is simply performed with the warm product
from the second inclined fluidized bed, handles the coal fines as well as
the normal fine dried coal.
Further, excess fines, removed from the exit gas stream of the first
inclined fluidized bed which are not burned in the combustor, can be
combined in this step and also formed into briquettes.
EXAMPLE 1
In order to dry coal, it is necessary first to investigate its
characteristics in order to determine the necessary temperature settings
for the inclined fluidized bed operations. In this test of the process two
crushed coals were employed: Eagle Butte from Campbell County, Wyoming,
and Usibelli from near Healey, Alaska. The feed coals were crushed to
minus 590 microns (minus 28 mesh) to produce an average particle diameter
of 70 microns for the Eagle Butte coal and 80 microns for the Usibelli
coal by wet screen analysis. Since wet coal fines tend to aggregate during
dry screening, wet screen analysis was employed to better characterize the
fines distribution. FIG. 3 shows the particle size distributions obtained
for these coals. Both coals are high-moisture subbituminous coals with "as
received" moisture contents of 29% and 22% for the Eagle Butte and
Usibelli coals, respectively. Coincidentally, both coals have a heating
value of 8470 Btu/lb. Table 1 gives proximate, ultimate, and heating value
analyses of the two coals.
Controlled tests of the rate of volatile loss from the coals as they were
heated at different heating rates are summarized in FIGS. 4 and 5. The
heating rate parameters on these graphs do not significantly affect the
results. In all cases the moisture is effectively removed by 200.degree.
C.
TABLE 1
______________________________________
Results of Chemical Analyses of Feed Coals
Analysis Eagle Butte Usibelli
______________________________________
Proximate (wt % as received)
Volatile Matter 30.9 36.4
Fixed Carbon 35.2 33.3
Ash 4.7 8.3
Moisture 29.2 22.0
Ultimate (wt % on dry basis)
Carbon 67.4 61.5
Hydrogen 5.1 5.2
Nitrogen 0.9 0.9
Sulfur 0.6 0.2
Oxygen 19.4 21.6
Ash 6.6 10.6
Heating value, Btu/lb
8470 8470
______________________________________
At higher temperatures gases other than water are emitted as pyrolysis
becomes important. Further gas analysis by component indicated that
hydrogen gas has maximum rates of evolution just above 400.degree. C.
Methane has a broader evolution peak with a maximum near 500.degree. C.
Ethene has a maximum rate of evolution near 400.degree. C. but also
evolves at a lower rate to 800.degree. C. Carbon dioxide has a broad
evolution profile starting near 100.degree. C. and extending to
1000.degree. C. with a maximum near 400.degree. C. Hydrogen is not formed
in significant amounts below 500.degree. C. These results are valid for
both coals. These conversion studies indicate that for both coals
significant pyrolysis conversion starts at near 250.degree. C. with
predominately carbon dioxide formed as the gaseous product below
400.degree. C.; however, as the carbon dioxide forms, these pyrolysis
reactions will also produce considerable liquid tar.
From the above information the preferred embodiment optimum operating
conditions are to keep the bed temperature below 200.degree. C.
(392.degree. F.) for only drying. This will evolve moisture without
allowing any significant pyrolysis to occur. Then rapid heating to near
350.degree. C. (662.degree. F.) will evolve carbon dioxide and mobilize
tar. Quenching to below 250.degree. C. (482.degree. F.) will stop the
pyrolysis, and slow the flow of the tar.
A series of cold flow experiments were run to determine the solids
residence time relationship to the gas-flow conditions with the slope of
the inclined fluidized bed as a parameter. If too low a gas velocity is
employed, the material will plug the inclined fluidized bed. The
correlation was made using a solid Reynolds number thus:
N.sub.RE =[D.sub.S V.sub.G P.sub.S ][u.sub.G ].sup.-1 ;
where N.sub.RE is the solids Reynolds number, D.sub.S is the average
diameter of the solid particles, V.sub.G is the fluidizing gas velocity,
P.sub.S is the solid particles density, and u.sub.G is the gas viscosity.
Units are appropriately picked to make this solids Reynolds number
dimensionless. FIGS. 6 and 7 show the results of these cold-flow test
correlations. These allow operating conditions to be rapidly obtained for
a wide range of process conditions.
EXAMPLE 2
With the previous information obtained in Example 1, bench drying runs were
made at various slopes of the inclined fluidized bed. The feed rate was
approximately ten pounds per hour, controlled by a mechanical feeder, for
these small scale tests, and carbon dioxide from the process was not
recycled, but instead a separate pressured supply of carbon dioxide was
used. Tables 2 and 3 give the results for a series of four hour runs with
an occasional twelve hour run utilized. The experimental yield values are
presented as percentages of the total feed coal as summarized in Table 1.
The product coal can be safely handled in a number of ways including
briquetting, direct bagging, transfer by mechanical or other means to a
storage area, or even as feed stock for additional coal processing.
It is evident that the product coal has been dried to a very low moisture
content for in all instances the moisture content was below 1.5%. The
heating values of the Eagle Butte dried product coals tested in the range
of 11,800 to 12,600 Btu/lb. Compared to the feed value of 8,470 Btu/lb,
this is a significant enhancement in product value. Similar improvement
would be expected for the Usibelli dried coal product from the information
shown in Table 3. Additionally, the process stability allowed operation
over an extended time period.
To further test the characteristics of the product coal, moisture
reabsorption, dust content, and spontaneous heating tests were performed.
TABLE 2
__________________________________________________________________________
Summary of Experimental Yields for IFB Bench-Scale
Drying Tests using Eagle Butte Feed Coal
Average
Reactor
Gas to
Dryer Experimental Yield %:
Slope,
Solids,
Temperature, Entrained
degrees
lb/lb
.degree.F.
Product
Gas
Solids Water
__________________________________________________________________________
3 4.9 589 29.6 4.7
35.0 28.0
3 2.7 531 57.0 2.5
11.6 28.2
.sup. 3.sup.a
3.9 695 36.7 8.8
28.4 28.9
6 2.7 595 34.0 2.2
38.5 27.2
6 4.0 599 38.3 3.3
35.3 21.9
6 4.1 623 58.0 2.7
20.5 20.9
6 2.5 666 50.7 7.5
12.3 26.9
.sup. 6.sup.a
3.0 684 47.9 10.1
13.4 26.1
9 4.6 617 39.5 4.1
32.0 24.1
9 3.6 589 47.4 5.5
16.1 27.1
9 2.3 588 57.0 5.8
7.7 27.2
9 4.8 692 21.0 7.6
40.9 26.9
.sup. 9.sup.a
1.5 611 52.6 5.7
11.1 29.1
12 1.4 603 55.9 3.6
13.7 25.5
12 1.3 649 55.9 7.1
6.7 26.1
12 2.3 682 45.5 9.2
15.1 27.8
15 1.4 645 55.8 4.8
9.3 27.6
15 1.4 377 63.6 0.9
10.1 23.9
15 0.7 589 -- -- -- --
15.sup.a
1.4 731 52.8 15.1
8.7 20.4
__________________________________________________________________________
.sup.a Experiment of nominally 12hr duration
TABLE 3
__________________________________________________________________________
Summary of Experimental Yields for IFB Bench-Scale
Drying Tests using Usibelli Feed Coal
Average
Reactor
Gas to
Dryer Experimental Yield %:
Slope,
Solids,
Temperature, Entrained
degrees
lb/lb
.degree.F.
Product
Gas
Solids Water
__________________________________________________________________________
3 2.6 494 70.9 6.9
9.3 13.4
3 3.4 705 50.6 15.0
14.9 17.2
3 3.7 690 33.1 14.8
31.3 18.1
3 3.4 605 49.7 10.6
20.1 18.7
.sup. 3.sup.a
4.0 611 54.2 8.3
15.3 20.5
6 2.7 690 53.9 13.3
13.6 17.3
6 2.1 675 52.8 17.2
6.2 20.0
6 3.3 695 56.0 14.0
7.0 19.6
6 2.8 564 64.9 5.9
8.0 18.8
.sup. 6.sup.a
2.6 664 55.9 13.9
11.8 16.6
9 2.6 637 55.7 9.2
10.4 22.1
9 2.7 571 43.9 6.6
27.7 20.0
9 1.9 603 64.9 8.0
5.4 21.7
9 3.8 707 44.1 12.8
22.3 18.6
.sup. 9.sup.a
1.9 632 60.9 10.2
10.2 17.8
12 1.5 632 66.0 7.4
8.6 18.4
12 1.3 653 63.7 7.7
10.0 17.9
12 2.3 692 58.5 12.1
9.9 15.8
15 1.3 648 66.6 7.2
7.1 20.0
15 1.4 364 69.3 3.7
5.5 19.3
15 0.7 594 -- -- -- --
.sup. 15.sup.a
1.3 752 60.3 15.3
6.3 15.4
__________________________________________________________________________
.sup.a Experiment of nominally 12hr duration
The moisture reabsorption test exposed samples of product coal to 95%
relative humidity at 30.degree. C. for five days. Typical results were
that the new level of equilibrium moisture after reabsorption was
approximately half that of the feed coal. The higher the average drying
temperature, the lower the new equilibrium moisture value became. In
actual instances 95% relative humidity may not always be encountered and
lower values better represent more realistic conditions. At 50% relative
humidity at 30.degree. C. for five days, the new equilibrium moisture
level was only about one-third that of the feed coal, and indicated the
success of the pyrolysis tar mobilization and quenching to prevent
moisture reabsorption.
Dust tests were performed using opacity meter measurements on product
samples of both coals. These test results confirmed that the dried coal
products contained very low levels of dust compared to the feed samples.
Spontaneous heating test were run under the standard conditions: 70.degree.
C. starting temperature with heating exposed to 160 cc/min oxygen
saturated with moisture. Ignition time or a 300.degree. C. coal
temperature ended each test. Table 4 gives the results which show that the
product coal self-heats quicker by a factor of two to three when compared
to the feed. This produces a better product combustion for future use but
also makes the final carbon dioxide pore treatment important for storage
safety.
A further verification of the process is that the bed temperature shown in
Tables 2 and 3, which is an average of several test positions, falls
generally in the range of the previously determined expected value of
approximately 350.degree. C. (662.degree. F.).
It is noted that although some bed inclination angles would be preferred
because of lower fines carry-over, the drying operation can be
successfully operated over a wide range of such angles.
TABLE 4
__________________________________________________________________________
Effect of Drying Conditions on Surface Area and Self-Heating
Characteristics
Self-heating
Surface
Time, min,
Test Reactor
Drying
Sample
Area to reach
Coal Type
Number
Slope
temp, .degree.F.
Location
m.sup.2 /g
200.degree. C.
__________________________________________________________________________
Eagle Butte
-- -- -- Avg. Feed
4.1 160
D-2 3 586 Product
4.8 145
D-30 3 531 Product
4.7 70
D-31 3 695 Product
4.2 45
D-37 6 684 Product
3.5 --
D-39 9 611 Product
3.0 75
D-53 15 731 Product
3.2 60
Usibelli
-- -- -- Avg Feed
1.7 >150
D-29 3 494 Product
0.7 130
D-32 3 705 Product
0.9 40
D-35 3 611 Product
0.9 75
D-36 6 664 Product
1.9 52
D-38 9 631 Product
1.4 60
D-52 15 752 Product
2.3 50
__________________________________________________________________________
EXAMPLE 3
A series of larger test were performed on a pilot plant process system that
was designed for approximately 100 pounds per hour feed rate of coal. This
feed coal was Eagle Butte with the properties given in Example 1. The
system was designed for mild coal gasification, and the drying aspects
were only the first part of the process; however recycle carbon dioxide
was employed. Therefore, two inclined fluidized beds were employed; the
first was principally a coal dryer, the second the mild coal gasification
unit. The results shown in FIG. 8 represents approximately a 24 hour pilot
plant run for the first inclined fluidized bed and gives comparable
results to the previous smaller scale experiments. In this instance the
inflection point on the bed temperature curve occurred at approximately
the midpoint of the bed; thus, indicating the start of significant
pyrolysis forming carbon dioxide.
Since the product coal was normally not separately removed but continued
directly on to mild coal gasification, the drying bed temperature was not
raised to the pyrolysis tar mobilization temperature. Nearly complete
moisture removal, however, was easily obtained as shown in FIG. 8. This
drying curve well illustrates the characteristic sections associated with
free, physically bound, and chemically bound moisture.
The test parameters for the illustrated number 117 run were: coal feed
rate, 119 lb/hr; coal residence time, 3 min; recycle gas flow, 92 scfm;
fluidizing gas temperature, 540.degree. F.; dryer zone temperatures,
.degree.F.: No. 1, 128; No. 2, 151; No. 3, 284.
The recycle carbon dioxide generally tested out at better than 95%, after
moisture and fines removal from the dryer exit gas, even after many hours
operation of the pilot plant. For this run the dryer produced 5.5% fines,
29.8% moisture, and 0.9% gas, with a basis of 100% for the feed and all
percentages are by weight. It is to be noted that the percentage of fines
as presented represents the fines produced only in the dryer; for these
pilot plant operations the feed coal had had its fines significantly
removed before processing.
The product coal can be safely handled in an appropriate manner as
indicated in Example 2.
It is noted that this feed coal in Table 1 analyzed at 29.2% moisture;
therefore, essentially complete removal was obtained.
The foregoing description of the specific embodiments will so fully reveal
the general nature of the invention that other can, by applying current
knowledge, readily modify and/or adapt for various applications such
specific embodiments without departing from the generic concept, and
therefore such adaptations are modifications are intended to be
comprehended within the meaning and range of equivalents of the disclosed
embodiments. It is to be understood that the phraseology or terminology
herein is for the purpose of description and not of limitation.
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